Science Buddies: "Ask an Expert"

Hey Science Budds,

This year I am doing the science buddy project: Using Daphnia to Monitor Water Toxicity.
I have already completed my experiment using:
2 grams of sediment in each container with my independent variables of 200mL, 300mL, and 400mL of spring water
placing 30 daphnia magna in each container
i completed 5 trials for each quantity of water plus a control (haha yup a lot of daphnia)

Can you please explain to me how I'm exactly supposed to calculate the LC50 with my numbers because it does not explain clearly.

Thank You and I hope you had an awesome christmas!,
ash

Hi Ashley!
I did a quick google search, and it seems that most people use computer programs to determine the LC50, although here are some websites you might want to check out-
http://www.ccohs.ca/oshanswers/chemicals/ld50.html
http://www.tpub.com/content/ArmyDOE/tre ... -10292.htm
http://www.agls.uidaho.edu/etox/resources/probit.pdf
Hope this helps, enjoy the holidays!

In order to determine the LC50, you first need to figure out the concentrations of sediment, then graph them against the mortality. Have a computer fit a best-fit line to the graph, then find where the line crosses the 50% mortality mark.

Does that make sense? If you don't have a computer program that will fit a best-fit line, you can draw one yourself.

I guess I'l have to draw it out the LC50 graph myself..

but how do i calculate the sediment concentration?
i did 2 grams of sediment and 200 mL with 30 daphnia in each as one example

-Ashley

I guess I'l have to draw it out the LC50 graph myself..

but how do i calculate the sediment concentration?
i did 2 grams of sediment and 200 mL with 30 daphnia in each as one example

-Ashley

There are many different units of concentration, but given your measurements I would suggest using g/mL--e.g. dividing the amount of sediment in grams by the amount of water in milliliters. This will give you your concentrations.